Increasingly, older elevator systems are modernized by retrofitting them with computer-based controls. Older systems frequently use low power relay logic to control elevator operations, such as car dispatching, and other relay controls that control high power operations, for instance, motor and brake operations. Usually, low power relay logic circuits are replaced by a computer during the modernization, but the "power" relays, those that control current to the high power equipment, are retained over far more expensive high power solid switching devices, because power relays are inexpensive, very reliable, and easy to service (i.e., replace) and, therefore, are retained whenever possible.
Following common practice, a suppressor diode is placed across these relay coil terminals to protect the relay drive from high voltage surges when the relay coil is operated. The diode "clamps"--to the diode's forward voltage--the voltage drop across the coil when the current is turned off (when the relay is deenergized). Self induced current, caused by the self-induced voltage from the collapsing coil current flows through the suppressor diode. In older systems, that current was switched on and off by low power control relays, and in those systems, if the suppressor diode opened, the voltage across the coil could be very high (coil voltage=L(coil inductance) di/dt), perhaps several thousand volts, which would produce arcing across the drive relay contacts to allow the coil current to continue. The resulting very gradual contact wear (pitting) could be spotted easily during periodic maintenance, at which point the suppressor diode would be replaced, perhaps along with the relay or the contacts.
Failure of the suppressor diode, however, can be "lethal" to a solid-state relay driver over time. These drivers often consist of low power solid-state output devices that typically can only tolerate a limited number of modest output over-voltage surges and voltage reversals before the device eventually fails suddenly and completely. Knowing that the suppressor diode has failed can be especially important in servicing an elevator system in which power relays are controlled by a solid-state device, such as a microprocessor with a solid-state relay drive output.
In some related applications, relay operation is monitored by sensing the relay coil voltage to determine the time of activation and deactivation of the relay. For instance, the relay coil terminals may be connected to the input port of a computer that controls sequences based on sensing operation of a relay. These input circuits are also very sensitive to voltage spikes and reversals on the relay coil.